<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
<div id="app">
  <input type="text" v-model="username">
</div>

<script src="./lib/vue-2.6.12.js"></script>
<script src="./lib/jquery-v3.6.0.js"></script>

<script>
    const vm = new Vue({
        el: '#app',
        data: {
            username: 'admin',
        },
        // 所有的侦听器，都应该被定义到 watch 节点下
        watch: {
            // 定义对象格式的侦听器
            username: {
                // 侦听器的处理函数
                handler (newVal, oldVal) {
                    console.log(newVal, oldVal)
                },
                // immediate 选项的默认值是 false
                // immediate 的作用是：控制侦听器是否自动触发一次！
                immediate: true,
            },
        },
    })
</script>
</body>

</html>
